The original author of this mod is Davidee. I have simply updated it from Minecraft 1.7.10 to 1.12.2. His thread for this mod can be found on the Minecraft Forums.
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This is a library that allows modders to create more advanced GUIs through the use of Containers. While it is not as powerful as GuiAPI, creating a GUI is done in a vanilla-style way and thus is very easy to do. For modders interested in learning how to use this library, you can take a look at my other projects on GitHub.
The library has functioning containers, scrollbars, textfields, buttons, sliders, labels, and more.